1. Field of the Invention
The present invention relates to deployable satellite antennas and, more particularly, to the boom structure that deploys, aligns and accurately holds the parabolic reflector (and/or subreflector) of the antenna to a satellite or another antenna element, and to ensuring accuracy of boom-to-reflector alignment on deployment of the antenna.
2. Discussion of the Related Art
One type of directional antenna commonly used in space based communications is the parabolic antenna. That antenna comprises a parabolic reflector and a microwave feed positioned at the focal point of the antenna. Another type of directional antenna that has achieved wide acceptance in the foregoing application is the dual reflector or Cassegrain antenna, which contains two reflectors, a parabolic reflector and a hyperbolic sub-reflector, the reflecting surfaces of which may be either concave or convex in shape.
Space based communications links typically require directional antennas that are deployable. The antenna construction and associated supports are articulated and fold-up for stowage in or on the satellite for transport into orbit. Once the satellite attains the correct orbit, the antenna is unfolded on command from a compact stowed condition and is deployed for operation, establishing a communication link.
To accomplish that a deployable antenna includes a boom (or booms), an arm that carries the reflector (or reflectors) from the stowed position on a satellite to the deployed position, setting up the antenna, and holds the reflector in that position thereafter. In the case of a space based deployable dual reflector antenna each of parabolic and hyperbolic reflectors is attached to a respective boom which positions and supports those reflectors in respective deployed positions. In a reflector antenna, the boom is carefully aligned and bolted to the reflector; and in the dual reflector antenna each reflector is carefully aligned and bolted to the respective reflector.
Many spacecraft applications require rigid, low-weight, and thermally stable components. Specifically, current spacecraft antenna applications require high precision reflector contours (RMS 0.001 to 0.002 inch) in addition to low thermal distortion and therefore, feature a variety of very complex configurations requiring lightweight, thermally stable composite materials. Bolting two parts together in such a precision assembly is problematic. The bolts must be torqued with care to the proper tightness to ensure that the two pieces cannot become detached during the ride into space or thereafter in the wide range of temperature extremes encountered in space, a range of about ±250 degrees Fahrenheit.
In torquing the attaching bolts it is possible to distort the surface of the reflector, and force the surface to depart from the high precision required, either initially or later when the antenna is deployed in space and encounters the known range of temperatures in that environment. Of necessity the bolts may be of a different material than the boom and possess a different characteristic of thermal expansion (and contraction). When exposed to a temperature extreme, because of the different thermal characteristics the bolts could become over-torqued and physically distort the reflector.
Anticipating the foregoing potential problem with prior antennas, typically, preflight checks are made of distortion. The entire antenna, including the boom or booms, are placed in a thermal chamber and checked for distortion over the anticipated thermal range of operation in space, although remaining subject to the effect of gravity. If the antenna fails the test, the entire antenna construction may need to be repeated. As is appreciated, the foregoing is a time consuming and expensive process necessitated by the inability or great difficulty and greater expense to send a repair crew into space to repair or replace a defective antenna.
As newer antennas have become larger and larger in size it becomes necessary to build larger and larger thermal chambers to implement a thermal test, which adds to the expense of developing an antenna. Alternatively one must forego thermal testing and bear the attendant risks if neither manufacturer nor customer wishes to bear the expense of thermal testing. The foregoing poses a problem to the manufacturer and customer who would each prefer to avoid the cost and risk.
As an advantage, the present invention avoids both the foregoing cost and risk by eliminating the bolts, the bolting, the testing, and the risk of thermally induced physical distortion of the reflector by eliminating attaching devices of materials that have thermal characteristics that differ significantly from that of the reflector.
A recent innovation in the construction of parabolic and hyperbolic reflectors is the composite isogrid reflector structure presented in U.S. Pat. No. 6,064,352 to Silverman et al (the '352 patent), granted May 16, 2000 and assigned to TRW Inc., the assignee of the present invention. The reflector construction of the '352 patent provides a reflector of high stiffness and lightweight, which are very desirable properties for space based antennas. Employing integral reinforced interlocked parabolically curved ribs connected in triangular isogrid patterns, a parabolic profile is defined collectively by the edges of the ribs on a side of the grid (or in the case of a sub-reflector a hyperbolically profile is defined collectively by the edges of the grid). The foregoing grid is permanently bonded to a thin curved reflective sheet, referred to as the facesheet, that serves as the (or hyperbolic) reflecting surface of the reflector. The isogrid structure adds strength and stiffness to the facesheet. The present invention takes advantage of the foregoing innovation and, accordingly, the applicants refer to and incorporate here within the content of the '352 patent.
Accordingly, a principal object of the present invention is to improve the design of deployable high precision parabolic antennas.
A further object of the invention is to minimize the occurrence of surface distortion in the reflectors of space based deployable antennas as a result of wide swings of temperature.
An additional object of the invention is to eliminate materials that possess significantly different thermal characteristics than the reflector of a space based deployable antenna from the boom to reflector attachment interface.
A still additional object of the invention is to eliminate any necessity for bolts to attach a deployable reflector to a boom in a deployable antenna.
A still further object of the invention is to reduce the cost of developing deployable high precision reflector antennas for space based application.
And an ancillary object of the invention is to provide a new design for a space based deployable dual reflector antenna.